The present invention relates to an elliptic gear or an unequal velocity ratio gear.
If the ratio of angular velocities of two shafts having a constant center distance could be continuously changed by a pair of gears, the torque at the input or output side would change in accordance with the velocity ratio so that the transmission device generally composed of many gears could be replaced by a gear unit having a simpler structure. The unequal velocity ratio gear of this kind, as known, is exemplified by the elliptic gear.
A pitch circle of the elliptic gear is generally expressed in polar coordinates by the following formula: ##EQU1## Here: R, the radius of the pitch circle of the elliptic gear; l.sub.n, one half of the straight chord of a reference ellipse; e.sub.n, the eccentricity of the reference ellipse; n, the degree of the ellipse; and .phi., the angle of rotation of the elliptic gear.
FIGS. 5A to 5D are contour diagrams showing examples of the elliptic gears which are determined on the basis of the above-specified formula. FIG. 5A illustrates the gear for n=1; FIG. 5B that for n=2; FIG. 5C that for n=3; and FIG. 5D that for n=4.
In the prior art, examples of the gear unit employing the elliptic gear are disclosed in Japanese Patent Laid-Opens Nos. 266866/1986 and 270863/1987.
As could be easily understood from the aforementioned formula and FIGS. 5A to 5D, the elliptic gear has no portion, in which the radius from the center of rotation is constant, so that the rotational angular velocity ratio of two shafts coupled to each other through the paired elliptic gears changes at all times. This changing behavior is explicitly illustrated in the diagrams of angular velocity ratio presented in the above-specified Laid-Opens.
Now, the desire of holding the velocity ratio constant within a predetermined range of rotational angle while achieving an unequal velocity ratio outside that range cannot be obtained by the aforementioned paired elliptic gears only. Therefore, the inventions disclosed in the above-specified Laid-Opens have adopted a plurality of elliptic gears and a mechanism for changing their connecting relations. Thus, according to the constructions, as disclosed, the angular velocity ratio can be constant within the predetermined angular range, but the construction of the system inherently become complicated. On the other hand, the gear unit of the prior art is constructed to effect the equal velocity rotations and the unequal velocity rotations by interchanging the connecting relations of the gears. This construction will cause such a disadvantage and limit the application that the change in the angular velocity ratio, i.e., the acceleration at the instant when the rotational motions change between the equal velocity rotations and the unequal velocity rotations inherently become abrupt.